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Q+ Hangout: Stephanie Wehner

Posted on 21 September, 2012 | 2 Comments

Here are the details for the next Q+ hangout.

Date: 25th September 2012

Time: 10am British Summer Time

Speaker: Stephanie Wehner (Singapore)

Title: Uncertainty and the generation of randomness

Abstract: We consider the creation of classical randomness by making measurements on a largely unknown quantum system. We show a close relations between measurements that are good at this task and measurements that satisfy strong uncertainty relations. Finally, we mention an application to quantum cryptography.

Please note the unusual start time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

or visit our website http://qplus.burgarth.de

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Rerun: Caslav Brukner’s Q+ hangout

Posted on 4 September, 2012 | Leave a comment

We are rerunning Caslav Brukner’s Q+ hangout due to problems with the livestream and video recording in July. Note the earlier than usual starting time.

Date: 18th September 2012

Time: 12noon British Summer Time

Speaker: Caslav Brukner (University of Vienna)

Title: Quantum correlations with indefinite causal order

Abstract:

In quantum physics it is standardly assumed that the background time or definite causal structure exists such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. We develop a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (“causal game”) that is impossible if the operations are ordered according to a fixed background time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

or visit our website http://qplus.burgarth.de

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Q+ Hangout: Francesco Buscemi

Posted on 13 August, 2012 | Leave a comment

Here are the details for the next Q+ hangout.

Date: 28th August 2012

Time: 2pm British Summer Time

Speaker: Francesco Buscemi (Nagoya University)

Title: All entangled quantum states are nonlocal: equivalence between locality and separability in quantum theory

Abstract:

In this talk I will show how, by slightly modifying the rules of nonlocal games, one can prove that all entangled states violate local realism.

As it is well known, Bell inequalities, which are used to test the violation of local realism, can be equivalently reformulated in terms of nonlocal games (namely, cooperative games with incomplete information) played between one referee and two (or more) players, these latter being separated so to make any form of communication between them impossible during the game. Quantum nonlocality is that property of quantum states that allows players sharing them to win nonlocal games more frequently than the assumption of local realism would imply.

However, as Werner proved in 1989, not all quantum states enable such a violation of local realism. In particular, Werner showed the existence of quantum states that cannot be created locally (the so-called “entangled” states) and, yet, do not allow any violation of local realism in nonlocal games. This fact has been since then considered an unsatisfactory gap in the theory, attracting a considerable amount of attentions in the literature.

In this talk I will present a simple proof of the fact that all entangled states indeed violate local realism. This will be done by considering a new larger class of nonlocal games, which I call “semiquantum,” differing from the old ones merely in that the referee can now communicate with the players through quantum channels, rather than being restricted to use classical ones, as it was tacitly assumed before. I will then prove that one quantum state always provides better payoffs than another quantum state, in semiquantum nonlocal games, if and only if the latter can be obtained from the former, by local operations and shared randomness (LOSR). The main claim will then follow as a corollary.

The new approach not only provides a clear theoretical picture of the relation between locality and separability, but also suggests, thanks to its simplicity, new experimental tests able in principle to verify the violation of local realism in situations where previous experiments would fail.

Based on http://arxiv.org/abs/1106.6095

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

or visit our website http://qplus.burgarth.de

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Q+ Hangout: Caslav Brukner

Posted on 12 July, 2012 | Leave a comment

Here are the details for the next Q+ hangout.

Date: 24th July 2012

Time: 2pm British Summer Time

Speaker: Caslav Brukner (University of Vienna)

Title: Quantum correlations with indefinite causal order

Abstract:

In quantum physics it is standardly assumed that the background time or definite causal structure exists such that every operation is either in the future, in the past or space-like separated from any other operation. Consequently, the correlations between operations respect definite causal order: they are either signalling correlations for the time-like or no-signalling correlations for the space-like separated operations. We develop a framework that assumes only that operations in local laboratories are described by quantum mechanics (i.e. are completely-positive maps), but relax the assumption that they are causally connected. Remarkably, we find situations where two operations are neither causally ordered nor in a probabilistic mixture of definite causal orders, i.e. one cannot say that one operations is before or after the other. The correlations between the operations are shown to enable performing a communication task (“causal game”) that is impossible if the operations are ordered according to a fixed background time.

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

or visit our website http://qplus.burgarth.de

→ Leave a comment

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Q+ Hangout: Scott Aaronson

Posted on 6 June, 2012 | Leave a comment

Here are the details for the next Q+ hangout.

Date: 19th June 2012

Time: 2pm British Summer Time

Speaker: Scott Aaronson (MIT)

Title: Quantum Money from Hidden Subspaces

Abstract:

Forty years ago, Wiesner pointed out that quantum mechanics raises the striking possibility of money that cannot be counterfeited according to the laws of physics. We propose the first quantum money scheme that is (1) public-key—meaning that anyone can verify a banknote as genuine, not only the bank that printed it, and (2) cryptographically secure, under a “classical” hardness assumption that has nothing to do with quantum money. Our scheme is based on hidden subspaces, encoded as the zero-sets of random multivariate polynomials. A main technical advance is to show that the “black-box” version of our scheme, where the polynomials are replaced by classical oracles, is unconditionally secure. Previously, such a result had only been known relative to a quantum oracle (and even there, the proof was never published). Even in Wiesner’s original setting—quantum money that can only be verified by the bank—we are able to use our techniques to patch a major security hole in Wiesner’s scheme. We give the first private-key quantum money scheme that allows unlimited verifications and that remains unconditionally secure, even if the counterfeiter can interact adaptively with the bank. Our money scheme is simpler than previous public-key quantum money schemes, including a knot-based scheme of Farhi et al. The verifier needs to perform only two tests, one in the standard basis and one in the Hadamard basis—matching the original intuition for quantum money, based on the existence of complementary observables. Our security proofs use a new variant of Ambainis’s quantum adversary method, and several other tools that might be of independent interest.

Based on http://arxiv.org/abs/1203.4740

Joint work with Paul Christiano

To view the seminar live, go to http://gplus.to/qplus at the appointed hour.

To stay up to date on future Q+ hangouts, follow us on:

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://www.facebook.com/qplushangouts

or visit our website http://qplus.burgarth.de

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Q+ Hangout: Joe Fitzsimmons

Posted on 21 May, 2012 | Leave a comment

Speaker: Joe Fitzsimons, Centre for Quantum Technologies, Singapore

Date: Tuesday 29th May 2012

Time: 14:00 British Summer Time

Title: Universal blind quantum computation

Abstract:

Blind Quantum Computing (BQC) allows a client to have a server carry out a quantum computation for them such that the client’s inputs, outputs and computation remain private. In this talk I will present a protocol for universal unconditionally secure BQC, based on the conceptual framework of the measurement-based quantum computing model. In this protocol the client only needs to be able to prepare single qubits in separable states randomly chosen from a finite set and send them to the server, who has the balance of the required quantum computational resources. This scheme has recently been implemented in a quantum optics setting. I will finish with a discussion of variants of the scheme allowing the client to detect deviations from the protocol by a malicious server.

To watch the seminars live, go to http://gplus.to/qplus at the appointed hour. You do not need a Google account to watch, but you do need one if you would like to be able to participate in the question and answer session at the end of the talk.

To stay up to date on the scheduled seminars you can visit our website or follow us on various social networks:

Our website: http://qplus.burgarth.de

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://facebook.com/qplushangouts

We also encourage you to suggest speakers for future talks. You can do so by adding them to the spreadsheet at http://bit.ly/qplussuggestions.

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Q+ Hangout: Vlatko Vedral

Posted on 30 April, 2012 | Leave a comment

Date: Tuesday 8th May 2012

Time: 14:00 British Summer Time

Speaker: Vlatko Vedral (University of Oxford/National University of Singapore)

Title: Using Temporal Entanglement to Perform Thermodynamical Work

Abstract: Here we investigate the impact of temporal entanglement on a system’s ability to perform thermodynamical work. We show that while the quantum version of the Jarzynski equality remains satisfied even in the presence of temporal entanglement, the individual thermodynamical work moments in the expansion of the free energy are, in fact, sensitive to the genuine quantum correlations. Therefore, while individual moments of the amount of thermodynamical work can be larger (or smaller) quantumly than classically, when they are all combined together into the (exponential of) free energy, the total effect vanishes to leave the Jarzynski equality intact. Whether this is a fortuitous coincidence remains to be seen, but it certainly goes towards explaining why the laws of thermodynamics happen to be so robust as to be independent of the underlying micro-physics. We discuss the relationship between this result and thermodynamical witnesses of spatial entanglement as well as explore the subtle connection with the “quantum arrow of time”.

Based on http://arxiv.org/abs/1204.5559 and possibly also http://arxiv.org/abs/1204.6168 if time permits

To watch the seminars live, go to http://gplus.to/qplus at the appointed hour. You do not need a Google account to watch, but you do need one if you would like to be able to participate in the question and answer session at the end of the talk.

To stay up to date on the scheduled seminars you can visit our website or follow us on various social networks:

Our website: http://qplus.burgarth.de

Google+: http://gplus.to/qplus

Twitter: @qplushangouts

Facebook: http://facebook.com/qplushangouts

We also encourage you to suggest speakers for future talks. You can do so by adding them to the spreadsheet at http://bit.ly/qplussuggestions.

→ Leave a comment

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